1. Field of the Invention
The invention is related to the field of magnetic recording disk drive systems and, in particular, to a current perpendicular to the plane (CPP) giant magnetoresistance (GMR) read sensor and associated methods of fabrication.
2. Statement of the Problem
Magnetic hard disk drive systems typically include a magnetic disk, a recording head having write and read elements, a suspension arm, and an actuator arm. As the magnetic disk is rotated, air adjacent to the disk surface moves with the disk. This allows the recording head (also referred to as a slider) to fly on an extremely thin cushion of air, generally referred to as an air bearing. When the recording head flies on the air bearing, the actuator arm swings the suspension arm to place the recording head over selected circular tracks on the rotating magnetic disk where signal fields are written to and read by the write and read elements, respectively. The write and read elements are connected to processing circuitry that operates according to a computer program to implement write and read functions.
In a disk drive utilizing perpendicular recording, data is recorded on a magnetic recording disk by magnetizing the recording medium in a direction perpendicular to the surface of the disk. CPP sensors typically include a sensor stack comprising an antiferromagnetic (AFM) layer, a reference layer, a spacer layer, a free layer, and a cap layer. The sensor stack is electrically coupled between two shield layers. A hard bias layer is electrically coupled to an upper shield layer, and insulating material (such as alumina) electrically separates the hard bias material and the sensor stack. Current flows from the upper shield through the sensor stack into the lower shield at a uniform current density.
In metallic CPP read sensors, one problem is that spin torque excitations limit the amount of current in the free layer and the reference layer. The spin torque excitations therefore limit the signal that may be obtained from the read sensor. Spin torque excitations are essentially noise induced when a sensing current is above a critical magnitude. Thus, the sensing current passing through the read sensor is limited by the spin torque excitations of the free layer and the reference layer. Further, because the current density is uniform, any spin torque suppression techniques applied only to the free layer are inadequate, because the current density of the reference layer becomes the limiting factor of the read sensor.